Characterization of Methylammonium Lead Iodide Thin Films Fabricated by Exposure of Lead Iodide Layers to Methylammonium Iodide Vapor in a Closed Crucible Transformation Process

In this work, the characterization of methylammonium lead iodide (MAPI) layers fabricated with a modified two-step deposition technique under high vacuum is presented. Thereby, PbI2, deposited in an open sublimation process, is exposed to methylammonium iodide (MAI) vapor in a closed crucible. The fabricated layers are examined with scanning electron microscopy (SEM), X-ray diffraction (XRD), UV/VIS absorption spectroscopy, and photoelectron spectroscopy (PES). In addition, planar solar cells incorporating the MAPI layers are produced. The obtained XRD data show that MAPI with high perovskite phase purity can be fabricated in a broad substrate temperature range between 75 degrees C and 150 degrees C. The SEM measurements show that with increasing substrate temperature the morphology of the MAPI layers undergoes significant changes which can be separated into three distinct processes, taking place simultaneously: formation of the perovskite by incorporation of MAI into the PbI2 grains, recrystallization of the perovskite grains, and an Ostwald ripening like growth of the recrystallized grains. By the combination of the UV/VIS and in vacuo PES data, band diagrams for PbI2, MAI, and MAPI can be drawn which appear to be independent on the substrate temperature.